"Transformation toughening" is a phenomenon which occurs in certain bodies containing small particles or grains of a metastable phase material. The metastable phase leads to toughening when it undergoes a martensitic or other sufficiently fast and energetic transformation to a stable phase at or ahead of a crack tip in the body. Transformation toughening of ceramics is most well known in bodies containing metastable tetragonal zirconia particles or grains where the phase transformation is to a stable monoclinic zirconia phase. Similar transformation toughening has also been shown for certain materials containing hafnia and has been suggested as a possibility in some other ceramic systems.
In materials systems using tetragonal zirconia, transformation toughening is observed in two principal arrangements. The first is where a fine (typically submicron) tetragonal zirconia precipitate is formed in a cubic zirconia matrix. The cubic zirconia usually has a substantial grain size (e.g. 100 .mu.m) resulting from high temperature treatment to achieve a solid solution of zirconia and a stabilizing compound (typically CaO or MgO). The cubic zirconia with tetragonal precipitate is commonly referred to as partially stabilized zirconia (PSZ).
The second principal occurrence of transformation toughening with zirconia occurs in a material consisting of fine zirconia grains (typically about 0.5-2 .mu.m) primarily of tetragonal zirconia. This material is obtained by sintering very fine zirconia powder containing only sufficient stabilizer (usually Y.sub.2 O.sub.3) to achieve the desired tetragonal phase. This tetragonal material is referred to a "TZP".
Transformation toughening using tetragonal zirconia can also occur when fine tetragonal zirconia particles are placed into a compatible material matrix. The most important materials system of this type uses an alpha alumina matrix. This material is referred to as zirconia-toughened alumina (ZTA).
Transformation toughened materials have been shown to undergo an increase in flexural strength when the as-fired surfaces of the body are subjected to grinding. Flexural strength increases due to surface grinding are typically on the order of about 20% for zirconia toughened ceramics. This increase in strength has been attributed to transformation from tetragonal to monoclinic phase at the surface of the body. For example, see U.S. Pat. No. 4,067,745 to Garvie et al. Since the tetragonal to monoclinic phase transformation involves a volume expansion, the transformation to monoclinic at the surface results in a surface compressive stress which is responsible for the strength increase.
While the strength increase associated with machining transformation-toughened materials is desirable, machining is typically one of the most costly operations performed on ceramic materials. Accordingly, machining is normally performed only to achieve dimensional tolerances or surface finish characteristics. Further, in addition to cost considerations, machining may not be practical where the body is intricately shape or where machining would involve unacceptable deviation from dimensional tolerances. Thus, there is a need for a less costly, more widely applicable method of achieving the surface strengthening effect normally associated with the machining of transformation toughened materials.